A silica (SiO2) film formed by a vacuum process such as chemical vapor deposition (CVD) has been widely used as an interlayer dielectric for semiconductor devices and the like. In recent years, a coating-type insulating film called a spin-on-glass (SOG) film which contains a tetraalkoxysilane hydrolysate as the main component has also been used in order to form a more uniform interlayer dielectric. Along with an increase in the degree of integration of semiconductor devices, a low-relative-dielectric-constant interlayer dielectric called an organic SOG film has been developed which contains a polyorganosiloxane as the main component.
An organic SOG material (organic silica sol) is cured by causing silanol groups contained in the sol to undergo dehydration condensation by heating at 350 to 500° C. An insulating film which exhibits a dielectric constant, mechanical strength, and chemical resistance suitable for an interlayer dielectric for semiconductor devices can be formed by using the organic silica sol. On the other hand, since the reaction of the organic silica sol is a solid phase reaction, dehydration condensation proceeds to only a small extent due to diffusion control, whereby heating for a long time (e.g. at least about 30 minutes, and usually one hour or more) is required. In order to perform the heat treatment for a long time, a batch heat treatment furnace which can process a plurality of (usually 50 to 150) wafers at a time has been used to process spin-on low-dielectric-constant interlayer dielectrics. A semiconductor device for which a low-dielectric-constant interlayer dielectric is required is mainly a semiconductor device in the logic device field. A wiring manufacturing step for logic devices has been shifting to a single-wafer process in which wafers are processed one by one in a short time. This is because an ASIC or a custom IC, which is the main trend of the logic devices, is manufactured through small-amount multi-product production, and a single-wafer process has become the main trend of the manufacturing processes in order to improve the degrees of freedom of the manufacturing step.
A method has been proposed in which a composition containing a polysiloxane as the main component is cured using electron beams (U.S. Pat. No. 6,204,201 and European Patent No. 1122770). This method is characterized in that application of electron beams with heating causes not only condensation of silanols but also decomposition and activation of organic groups in an organic silica film, whereby another crosslinked structure such as Si—CHx-Si can be introduced. According to the method using electron beams, a film exhibiting low hygroscopicity and excellent mechanical strength can be obtained by applying electron beams usually for five minutes or less to allow a single-wafer process.
On the other hand, since electric charges accumulated due to application of electron beams may damage a transistor structure in an LSI, there are arguments both for and against the method of curing an interlayer dielectric composition using electron beams (E. Mickler et al., Proceedings of the International Interconnect Conference, p. 190, 2004, Miyajima et al., Proceedings of the International Interconnect Conference, p. 222, 2004).
As a method of curing an interlayer dielectric composition containing an organic silica sol as the main component in a short time without using electron beams, a method using ultraviolet radiation (UV) may be considered.
Technology other than the LSI interlayer dielectric technology is considered below. A technology of combining a silica sol and an alkoxysilane with a photoacid generator or a photobase generator which generates an acid or base upon application of ultraviolet radiation to promote condensation of silanols and alkoxides, thereby causing gelation of the silica sol has been known as an optical sol-gel technology, and applied to form optical waveguides and the like (e.g. JP-A-2000-109695). A silica film obtained by curing the composition using the photoacid generator or the photobase generator generally contains a large amount of residual silanols. Therefore, such a silica film exhibits high hygroscopicity and a high dielectric constant. The amount of water due to the residual silanols may be reduced by heating the gel obtained by applying ultraviolet radiation at about 250 to 500° C. for a period equal to or longer than a specific period (usually 30 minutes or more). However, this method is the same as the above method of curing the silica film by heating. The composition containing the photoacid generator or the photobase generator may pose a problem such as a decrease in insulating properties or deterioration of a wiring metal, since the photoacid generator, the photobase generator, or an acidic or basic substance generated therefrom serves as a charge carrier. This may make it difficult to ensure the quality of an insulating film for LSI semiconductor devices for which high insulation reliability is required.
Since a siloxane compound exhibits excellent transparency to ultraviolet radiation, the siloxane compound has been extensively researched as the main skeleton for F2 photoresists for which ultraviolet radiation with a wavelength of 157 nm is used (e.g. JP-A-2002-268226). This technology basically utilizes the principle of a chemically-amplified photoresist using a KrF or ArF light source although the siloxane is used as the backbone. In this technology, a photoacid generator generates an acidic substance upon application of ultraviolet radiation, and a chemical bond cleaved due to the acid produces a functional group such as a carboxylic acid which is readily dissolved in a basic developer. Specifically, this technology does not promote the crosslinking reaction of the silica sol using ultraviolet radiation.
Since the surface of an organic silica film cured by applying heat, electron beams, or the like exhibits high hydrophobicity, ultraviolet radiation may be applied to reduce the surface hydrophobicity (e.g. U.S. Pat. No. 6,383,913, JP-A-63-248710, JP-A-63-289939, JP-B-8-29932, and JP-A-2001-110802). The above technology is characterized in that the top surface of the organic silica film is oxidized using ozone produced by application of ultraviolet radiation in air to change the hydrophobic surface to a highly reactive hydrophilic surface such as a silanol. This modification is mainly performed to improve adhesion to a film formed in the upper layer.
As described above, technology of applying a polysiloxane resin solution or an organic silica sol solution to a substrate and applying ultraviolet radiation after forming a film has been widely studied. On the other hand, only limited technologies have been reported in which ultraviolet radiation is positively utilized to cure the organic silica sol for forming an interlayer dielectric for LSI semiconductor devices. These limited related-art technologies are disclosed in JP-A-3-30427, JP-A-1-194980, WO 03/025994, and US-A-2004/0058090.
JP-A-3-30427 discloses technology in which a solution prepared by dissolving a tetraalkoxysilane (e.g. tetraethoxysilane (TEOS)) in collodion is applied to a semiconductor substrate and irradiated with ultraviolet radiation in a nitrogen atmosphere to obtain a silicon dioxide film at a low temperature. This technology is characterized in that highly volatile TEOS is fixed using the collodion, and decomposition of the collodion and dehydration condensation of TEOS are promoted by applying ultraviolet radiation.
JP-A-1-194980 discloses technology in which an organosiloxane resin is applied to a substrate and irradiated with ultraviolet radiation with a main wavelength of 254 nm with heating at 200° C. or less, the surface of the organosiloxane film is oxidized by ozone produced by application of ultraviolet radiation, and the organosiloxane film is heated at 400° C. or more, particularly about 900° C. to obtain a densified silicon dioxide film.
WO 03/025994 and US-A-2004/0058090 disclose technology in which hydrogen silsesquioxane (HSQ) or methyl silsesquioxane (MSQ) is cured by applying ultraviolet radiation. In this technology, when ultraviolet radiation is applied to HSQ or an HSQ/MSQ cocondensate in the presence of oxygen, active oxygen (e.g. ozone) produced in the system promotes oxidation of Si—H in HSQ to form a silica film containing a large number of SiO2 bonds. These related-art documents state that the above method is also effective for MSQ and it is effective to cure MSQ in the presence of oxygen. Accordingly, it is considered that the main mechanism of the crosslinking reaction in the above technology is formation of an SiO2 bond due to active oxygen. As described above, it is difficult to form SiO2 bonds in a short time using other curing methods. The feature of the above technology is using ultraviolet radiation. A silica film formed according to the above technology exhibits a high modulus of elasticity and high hardness due to an increase in the number of SiO2 bonds. On the other hand, the hygroscopicity and the dielectric constant of the film are increased due to an increase in hydrophilicity. Accordingly, as a low-dielectric-constant interlayer dielectric for LSI semiconductor devices and its formation method, (a) an organic silica sol which does not contain a source of an ionic substance, a charge carrier, or a corrosive compound such as a photoacid generator, a photobase generator, or a photosensitizer and can be cured in a short time, (b) a method of curing an organic silica film which does not damage a transistor structure and is carried out using a single-wafer process, (c) an organic silica film which does not exhibit hygroscopicity and exhibits high hydrophobicity, and (d) an organic silica film exhibiting mechanical strength to such a degree that the organic silica film can withstand formation of a copper damascene structure have been demanded.
In particular, when forming a multilayer wiring structure for semiconductor devices, an insulating film formed is subjected to plasma etching and chemical treatment during processing. An insulating film obtained by the related-art technology exhibits insufficient plasma etching resistance, even if it exhibits a low relative dielectric constant and high mechanical strength.
Plasma damage during processing the insulating film is mainly caused by a phenomenon in which radicals produced by plasma remove CH3 from an Si—CH3 structure of a polysiloxane. A silyl radical secondarily produced when CH3 is removed from the Si—CH3 structure promptly reacts with an oxygen atom or an oxygen radical present near the silyl radical and attracts hydrogen to form a silanol group (Si—OH). The presence of the silanol group increases the hygroscopicity of the insulating film, whereby an increase in relative dielectric constant, deterioration in chemical resistance, and a decrease in electrical insulating properties occur.
As a method of improving plasma resistance, a method may be considered in which the absolute amount of Si—CH3 structure in the insulating film is merely increased so that a large amount of CH3 is removed in the top layer to form a densified layer in the top layer, thereby improving apparent plasma resistance and RIE resistance. However, there is a limit to introduction of the Si—CH3 structure group into the polysiloxane from the viewpoint of maintaining the performance of the insulating film, particularly the hardness and the modulus of elasticity. As a method of improving plasma resistance while maintaining the property balance of a low-dielectric-constant interlayer dielectric, a method of incorporating an Si—CH2—Si unit into a polysiloxane structure can be given. The Si—CH2—Si unit has resistance to abstraction reaction due to radicals in comparison with the Si—CH3 unit. By dispersing the Si—CH2—Si units in the film, reactive radicals serve as a barrier diffused in the film, whereby the plasma resistance of the entire film is improved.
As a material which contains the Si—CH2—Si unit and used to form a low-dielectric-constant interlayer dielectric, a composition containing a polycarbosilane or prepared by mixing a polysiloxane and a polycarbosilane has been proposed (JP-A-2001-127152).
This composition aims at improving heat resistance and hygroscopic resistance. However, since the polycarbosilane and the polysiloxane undergo microphase separation when forming a film, it is difficult to uniformly disperse the Si—CH2—Si units over the entire film. This results in formation of a portion with low plasma resistance, whereby an interlayer dielectric exhibiting low plasma resistance over the entire film is obtained.